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United States Patent |
5,764,669
|
Nagai
|
June 9, 1998
|
Semiconductor laser including disordered window regions
Abstract
A method for fabricating a semiconductor laser device includes successively
epitaxially growing a quantum-well structure active layer and a second
conductivity type Al.sub.r Ga.sub.1-r As first upper cladding layer on a
first conductivity type GaAs substrate, forming an SiO.sub.2 film on a
region in a vicinity of the laser resonator facet on the second
conductivity type first cladding layer, annealing, thereby absorbing Ga
from the second conductivity type first upper cladding layer to form and
diffuse vacancies to reach the quantum-well structure active layer,
thereby disordering the quantum-well structure active layer in a region in
the vicinity of the laser resonator facet. Therefore, it is possible to
form a window structure by disordering the quantum-well structure active
layer without generating crystal transitions. In addition, there is no
necessity of implanting Si ions so as to diffuse those ions to form a
window structure, and there arises no unlikelihood of disordering that
because the Si ions are trapped during their diffusion by crystal defects
formed by the ion implantation, whereby a semiconductor laser device
provided with a desired window structure can be produced with high
reproducibility.
Inventors:
|
Nagai; Yutaka (Tokyo, JP)
|
Assignee:
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Mitsubishi Denki Kabushiki Kaisha (Tokyo, JP)
|
Appl. No.:
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584200 |
Filed:
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January 11, 1996 |
Foreign Application Priority Data
Current U.S. Class: |
372/46.015; 372/45.01 |
Intern'l Class: |
H01S 003/19 |
Field of Search: |
372/45,46,49,43,44
438/36-37
|
References Cited
U.S. Patent Documents
4827483 | May., 1989 | Fukuzawa et al. | 372/45.
|
4875216 | Oct., 1989 | Thornton et al. | 372/46.
|
5020068 | May., 1991 | Isshiki | 372/46.
|
5376582 | Dec., 1994 | Behfar-Rad et al. | 438/37.
|
5469457 | Nov., 1995 | Nagai et al.
| |
5577063 | Nov., 1996 | Nagai et al. | 372/46.
|
Foreign Patent Documents |
0213826 | Mar., 1987 | EP.
| |
1184974 | Jul., 1989 | JP.
| |
396290 | Apr., 1991 | JP.
| |
4103187 | Apr., 1992 | JP.
| |
4103186 | Apr., 1992 | JP.
| |
Other References
Itaya et al., "New Window-Structure InGaAlP Visisble Light Laser Diodes By
Self-Selective Zn Diffusion-Induced Disordering", IEEE Journal of Quantum
Electronics, vol. 27, No. 6, Jun. 1991, pp. 1496-1500.
Deppe et al., "Stripe-Geometry Quantum Well Heterostructure Al.sub.x
Ga.sub.1-x As-GaAs Lasers Defined By Defect Diffusion", Applied Physics
Letters, vol. 49, No. 9, Sep. 1986, pp. 510-512.
Ralston et al., "Room-Temperature Exciton Transactions In Partially
Intermixed GaAs/AlGaAs Superlattices", Applied Physics Letters, vol. 52,
No. 19, May 1988, pp. 1511-1513.
|
Primary Examiner: Bovernick; Rodney B.
Assistant Examiner: Phan; Luong-Quyen T.
Attorney, Agent or Firm: Leydig, Voit & Mayer, Ltd.
Claims
What is claimed is:
1. A semiconductor laser device comprising:
a first conductivity type GaAs substrate;
a first conductivity type Al.sub.x Ga.sub.1-x As (0<x<1) lower cladding
layer disposed on the substrate;
a quantum-well structure active layer comprising first conductivity type
Al.sub.z Ga.sub.1-z As (0<z<x) barrier layers and Al.sub.y Ga.sub.1-y As
(0<y<z) well layers, and having a disordered window region including Ga
vacancies disordering the window region, the disordered window region
being located proximate a laser facet, the quantum-well structure active
layer being disposed on the lower cladding layer and including a
striped-shaped region extending in a laser resonator direction, the
quantum-well structure active layer having disordered regions including Ga
vacancies on opposite sides of the stripe-shaped active region;
a second conductivity type Al.sub.r Ga.sub.1-r As (z<r<1) first upper
cladding layer disposed on the quantum-well structure active layer;
a GaAs surface-protecting layer disposed on the second conductivity type
first upper cladding layer;
a second conductivity type Al.sub.r Ga.sub.1-r As (z<r<1) second upper
cladding layer disposed on the surface-protecting layer;
a second conductivity type GaAs contact layer disposed on the second upper
cladding layer regions of the second upper cladding layer and the contact
layer not opposite the stripe-shaped active region being implanted with
protons and having a high resistance; and
electrodes disposed on the GaAs substrate and on the contact layer,
respectively.
Description
FIELD OF THE INVENTION
The present invention relates to a method of fabricating a semiconductor
laser device and a semiconductor laser device and, more particularly, to a
method of fabricating a semiconductor laser device that includes a window
structure in the vicinity of the laser facet and that enables a high
output operation, and a semiconductor laser device fabricated thereby.
BACKGROUND OF THE INVENTION
FIG. 5(a) is a perspective view of a semiconductor laser device, FIG. 5(b)
is a cross-sectional view taken along a line 5b--5b of FIG. 5(a), i.e., in
the resonator length direction of the semiconductor laser device, and FIG.
5(c) is a cross-sectional view taken along a line 5c--5c, i.e., in the
direction perpendicular to the resonator length direction of the
semiconductor laser device. In the figures, reference numeral 1 designates
an n type GaAs substrate. An n type Al.sub.x Ga.sub.1-x As (x=0.5) lower
cladding layer 2, 1.5-2 .mu.m thick, is disposed on the n type GaAs
substrate 1. A quantum-well structure active layer 3 comprising a
plurality of Al.sub.y Ga.sub.1-y As (y=0.05.about.0.15) well layers (not
shown) and Al.sub.z Ga.sub.1-z As barrier layers having an aluminum
composition z of 0.2.about.0.35 (not shown) is disposed on the n type
lower cladding layer 2. This quantum-well structure active layer 3
includes light guide layers (not shown) having the same composition as
that of the barrier layers and a thickness of about 35 nm at both sides
thereof, and it is constituted by three well layers about 10 nm thick and
two barrier layers of about 10 nm thick alternatingly laminated with each
other. Reference numeral 4a designates a p type Al.sub.r Ga.sub.1-r As
(r=0.5) first upper cladding layer 0.05-0.5 .mu.m thick, and numeral 4b
designates a p type Al.sub.r Ga.sub.1-r As (r=0.5) second upper cladding
layer, and the total thickness of the first upper cladding layer 4a and
the second upper cladding layer 4b is about 1.5.about.2.0 .mu.m. A p type
GaAs contact layer 5, 0.5.about.1.0 .mu.m thick, is disposed on the second
upper cladding layer 4b. Reference numeral 8 designates a proton
implantation region, numeral 9 designates an n side electrode, and numeral
10 designates a p side electrode. Reference numeral 15 designates a
silicon (Si) diffusion region formed by ion implantation and annealing and
numeral 20 designates a laser resonator facet. Reference numeral 3a
designates an active region of the active layer 3 which contributes to the
laser light emission and numeral 3b designates a window structure region
in the vicinity of the laser resonator facet. This semiconductor laser
device has a length in the laser resonator direction of 300.about.600
.mu.m and a width of about 300 .mu.m.
FIGS. 6(a)-6(e) are diagrams illustrating process steps in the method of
fabricating the prior art semiconductor laser device. In the figures, the
same reference numerals as in FIGS. 5(a)-5(c) designate the same or
corresponding parts. Reference numeral 11 designates a stripe shaped first
photoresist, numeral 14 designates a second photoresist, and arrows 23
indicate proton implantation, respectively.
A description is given of the fabricating method of the semiconductor laser
device with reference to FIGS. 6(a)-6(e).
A lower cladding layer 2, a quantum-well structure active layer 3, and a
first upper cladding layer 4a are successively epitaxially grown on an n
type GaAs substrate 1 in a wafer state. The cross-section of the wafer
after the growth is shown in FIG. 6(a). Next, photoresist is applied to
the surface of the contact layer 5 and it is patterned to form a stripe
shaped first photoresist mask 11 extending in what becomes the laser
resonator length direction and not reaching the position which becomes the
laser resonator facet. The interval between the photoresist mask 11 and
the position which becomes the resonator facet of the semiconductor laser
device is about 20 .mu.m, and the length of the stripe shaped photoresist
mask in the direction perpendicular to the laser resonator length
direction is 1.5.about.5 .mu.m.
Subsequently, ion implantation of Si is performed, reaching the active
layer 4, from the upper surface of the first upper cladding layer 4a
employing the photoresist mask 11 as an ion implementation mask, and the
photoresist mask 11 is removed. Then, the Si dose amount to the region
where the Si ions are to be implanted is set to 1.times.10.sup.13
.about.1.times.10.sup.14 cm.sup.-2. At the region below the photoresist
mask 11, no Si ions are implanted. Here, after removal of the photoresist
mask 11, annealing is carried out so as to disorder the active layer 3.
This is carried out because no disordering of the active layer 3 occurs
solely by the ion implantation; thermal processing is required to make the
Si atoms diffuse in the crystal. Generally employed as this thermal
processing is a method of annealing the wafer at a temperature above
700.degree. C. in an ambient with As pressure applied. As a result of this
annealing, the Si diffused region 15 is formed as shown in FIG. 6(c), and
the quantum-well structure active layer 3 in this region 15 is disordered.
A region in the vicinity of the laser resonator facet of the disordered
quantum-well structure active layer 3 becomes a window structure region
3b. The region other than the disordered region becomes the active region
3a.
Next, in the step of FIG. 6(d), after the second upper cladding layer 4b
and the contact layer 5 are successively epitaxially grown on the first
upper cladding layer 4a, the upper surface of the contact layer 5 is
covered by photoresist and it is patterned by the photolithographic
technique. In the step of FIG. 6(e), on a region where the stripe shaped
first photoresist mask 11 is formed, a stripe shaped second photoresist 14
extending in the laser resonator direction and having approximately the
same size as the first photoresist mask 11 is formed, and implantation of
protons is carried out from the upper surface of the contact layer 5
employing the resist 14 as a mask so that the implantation peak is
positioned in the second upper cladding layer 4b. Thereby, a region 8
where implantation of proton is carried out is formed in the contact layer
5 and the second upper cladding layer 4b, and this region 8, which is a
high resistance region, serves as a current blocking layer.
At last, after removal of the resist 14, a p side electrode 10 is formed on
the contact layer 5, an n side electrode 9 is formed on the substrate 1,
and a laser resonator facet 20 is formed by cleaving, thereby producing a
semiconductor laser device provided with a window structure as shown in
FIGS. 5(a)-5(c).
A description is given of the operation of the prior art semiconductor
laser device. When a plus voltage is applied to the p side electrode 10
and a minus voltage is applied to the n side electrode 9, holes are
injected into the quantum-well structure active layer 3 through the p type
contact layer 5, the p type second upper cladding layer 4b, and the p type
first upper cladding layer 4a, and electrons are injected into the
quantum-well structure active layer 3 through the n type semiconductor
substrate 1 and the n type AlGaAs cladding layer 2, and recombination of
electrons and holes occurs in the active region of the active layer 3,
thereby generating induced emission light in the active region 3a of the
quantum-well structure active layer 3. When light exceeding the waveguide
loss is generated by sufficiently increasing the injected amount of
carriers, laser oscillation occurs. Here, since the region 8 where
implantation of proton is carried out becomes high resistance because of
the proton implantation, no current flows through the p type contact layer
5 and the p type second upper cladding layer 4b in the proton implantation
region 8. In other words, a current flows through only the region where no
proton implantation is carried out.
A description is given of the window structure. Generally, the maximum
light output of an AlGaAs series semiconductor laser device that emits a
laser beam having a 0.8 .mu.m wavelength and that is employed as a light
source of an optical disc apparatus, such as compact disc (CD), is
determined by the light output at which facet destruction is generated.
The facet destruction is a phenomenon in which the crystal itself
constituting the semiconductor laser device is melted by the heat
generated due to the light absorption by the surface energy levels at the
facet region. Therefore, in order to realize high light output operation,
the device is required not to have facet destruction even with a high
light output. In order to realize that, it is very effective to provide a
structure that makes the facet region of the active layer not absorb the
laser beam, i.e., to provide a window structure that is "transparent" to
the laser beam. This window structure is obtained by providing a region
that has a higher energy band gap than the active region of the active
layer that emits the laser beam in the vicinity of the laser resonator
facet. In the prior art semiconductor laser device shown in FIG. 5(a),
since the active layer 3 comprises a quantum-well structure, the window
structure is formed by disordering of the quantum-well structure 3 by the
Si ion implantation 22 and the annealing. FIGS. 7(a) and 7(b) show a
profile of aluminum composition of the quantum-well structure active layer
3 before the disordering and a profile of aluminum composition of the
quantum-well structure active layer 3 after the disordering, respectively.
In FIGS. 7(a) and 7(b), the same reference numerals as in FIG. 1 designate
the same or corresponding parts. Reference numerals 30, 31, and 32
designate a well layer, a barrier layer, and a light guide layer,
respectively, of the active layer 3. In the figures, the ordinate
represents Al composition ratio and the abscissa represents position in
the crystal growth direction, of the lower cladding layer 2, the active
layer 3, and the upper cladding layer 4. Reference character Al2
represents Al composition ratio of the well layer 30, Al1 represents Al
composition ratio of the barrier layer 31 and the light guide layer 32,
and Al3 represents Al composition ratio of the active layer 3 after the
disordering, respectively. When silicon atoms (Si) are implanted into the
quantum-well structure active layer 3 shown in FIG. 7(a) and annealed, the
atoms constituting the well layer 30 and the barrier layer 32 are mixed
with each other, and the diffused region becomes a disordered region as
shown in FIG. 7(b). As a result, the Al composition ratio of the
disordered quantum-well structure active layer 3 becomes the Al
composition ratio Al3 that is approximately equal to the Al composition
ratio Al1 of the barrier layer 31 and the light guide layer 32, and the
effective energy band gap of the active layer 3 becomes approximately
equal to those of the barrier layer 31 and the light guide layer 32.
Accordingly, in the prior art semiconductor laser device shown in FIG.
5(a), the effective energy band gap of the disordered region of the
quantum-well structure active layer 3 becomes larger than the effective
energy band gap of the active layer 3 that is not disordered and serves as
the active region 3a, and the disordered region of the quantum-well
structure active layer 3 serves as a window structure that is
"transparent" to the laser light and the region of the quantum-well
structure active layer 3 in the vicinity of the laser resonator facet 20
serves as the window structure region 3b.
In the prior art semiconductor laser device having a window structure, the
quantum-well structure active layer 3 in the vicinity of the laser
resonator facet 20 is disordered by the diffusion of Si to form the window
structure region 3b. In this semiconductor laser device, however, in the
process of ion implanting Si in the fabricating process, a lot of crystal
defects are generated in the ion implanted semiconductor layer, whereby a
lot of crystal dislocations are generated in the first upper cladding
layer 4a and the active layer 3. This is because while the atoms
accelerated by a voltage are implanted in the crystal, they repeatedly
collide with atoms in the crystal, losing their energy and are finally
stopped, generating a lot of defects in the crystal. Although such crystal
transitions restore themselves to some degree in the annealing, they do
not restore themselves completely and the crystal dislocations partially
remain as they are. Since these crystal dislocation absorb the laser
light, even when the quantum-well structure active layer 3 is disordered
to increase its energy band gap to a value larger than that of the active
region 3a to form a window structure region 3b, it does not serve
effectively as a window structure.
When there are many crystal defects which would generate crystal
transitions, the silicon atoms themselves which are diffused by the
annealing are trapped at the crystal defects, thereby making the diffusion
difficult. This makes the disordering unlikely to occur and unable to
produce a semiconductor laser device with a desired window structure with
high reproducibility.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of fabricating
a semiconductor laser device that can form a window structure without
generating crystal transitions as well as with high reproducibility.
It is another object of the present invention to provide a semiconductor
laser device that has a window structure that is formed without generating
crystal transitions as well as with high reproducibility.
Other objects and advantages of the present invention will become apparent
from the detailed description given hereinafter; it should be understood,
however, that the detailed description and specific embodiment are given
by way of illustration only, since various changes and modifications
within the scope of the invention will become apparent to those skilled in
the art from this detailed description.
According to a first aspect of the present invention, a method for
fabricating a semiconductor laser device comprises forming successively a
first conductivity type Al.sub.x Ga.sub.1-x As (0<x<1) lower cladding
layer, a quantum-well structure active layer comprising an Al.sub.z
Ga.sub.1-z As (0<z<x) barrier layer and an Al.sub.y Ga.sub.1-y As (0<y<z)
well layer, and a second conductivity type Al.sub.r Ga.sub.1-r As (z<r<1)
first upper cladding layer on the front surface of the first conductivity
type GaAs substrate by epitaxial growth, forming a silicon dioxide
(SiO.sub.2) film on a region in the vicinity of the laser resonator facet
on the second conductivity type first cladding layer, annealing the
SiO.sub.2 film and the semiconductor layers formed by epitaxial growth,
and absorbing Ga from the second conductivity type first upper cladding
layer to form vacancies as well as diffusing the vacancies up to reaching
the quantum-well structure active layer, thereby disordering the
quantum-well structure active layer in the region in the vicinity of the
laser resonator facet, and after removal of the SiO.sub.2, regrowing
successively a second conductivity type Al.sub.r Ga.sub.1-r As (z<r<1)
second cladding layer and a second conductivity type GaAs contact layer on
the second conductivity type first upper cladding layer by epitaxial
growth. Therefore, there is no necessity of implanting Si ions, and it is
possible to form a window structure by disordering the quantum-well
structure active layer without generating crystal transitions. In
addition, there is no necessity of implanting Si ions so as to diffuse
those to form a window structure as well as there arises no unlikelihood
of disordering due to that Si ions are trapped during their diffusion by a
lot of crystal defects which are formed by the ion implantation, whereby a
semiconductor laser device provided with a desired window structure can be
obtained with high reproducibility.
According to a second aspect of the present invention, a method for
fabricating a semiconductor laser device comprises forming successively a
first conductivity type Al.sub.x Ga.sub.1-x As (0<x<1) lower cladding
layer, a quantum-well structure active layer comprising an Al.sub.z
Ga.sub.1-z As (0<z<x) barrier layer and an Al.sub.y Ga.sub.1-y As (0<y<z)
well layer, and a second conductivity type Al.sub.r Ga.sub.1-r As (z<r<1)
first upper cladding layer on a first conductivity type GaAs substrate by
epitaxial growth, forming a silicon dioxide (SiO.sub.2) film having a
stripe shaped opening with a prescribed width not reaching the laser
resonator facet extending in the direction becoming a laser resonator
length direction on the second conductivity type first upper cladding
layer, annealing the SiO.sub.2 film and the semiconductor layers formed by
epitaxial growth, and absorbing Ga from the second conductivity type first
upper cladding layer to form vacancies as well as diffusing the vacancies
up to reaching the quantum-well structure active layer, thereby
disordering the quantum-well structure active layer in the region in the
vicinity of the laser resonator facet, after removal of the SiO.sub.2
film, regrowing successively a second conductivity type Al.sub.r
Ga.sub.1-r As (z<r<1) second upper cladding layer and a second
conductivity type GaAs contact layer on the second conductivity type first
upper cladding layer by epitaxial growth, forming a resist film on a
region on the contact layer where the stripe shaped opening is formed, and
implanting protons to a depth not reaching to the quantum-well structure
active layer from above the contact layer, and after removal of the
resist, forming electrodes on the rear surface of the GaAs substrate and
on the upper surface of the contact layer. Therefore, there is no
necessity of implanting Si ions, and it is possible to form a window
structure by disordering the quantum-well structure active layer without
generating crystal transitions. In addition, there is no necessity of
implanting Si ions so as to diffuse those to form a window structure,
whereby a semiconductor laser device provided with a desired window
structure can be obtained with high reproducibility.
According to a third aspect of the present invention, the above-described
method further comprises after forming the SiO.sub.2 film, forming a
silicon nitride (Si.sub.3 N.sub.4) film on a region above the SiO.sub.2
film and the second conductivity type first upper cladding layer where the
stripe shaped opening is formed, and after disordering the quantum-well
structure active layer, removing the Si.sub.3 N.sub.4 film. Therefore, in
the annealing process, the surface roughness of the first upper cladding
layer can be prevented by preventing the slipping out of As from the
surface of the first upper cladding layer exposed to the stripe shaped
opening.
According to a fourth aspect of the present invention, in the
above-described method, the process of epitaxially growing the second
conductivity type Al.sub.r Ga.sub.1-r As (z<r<1) first upper cladding
layer is followed by epitaxially growing a GaAs surface protecting layer
subsequently performed thereto. Therefore, the oxidation of the regrowth
interface is prevented and the surface roughness of the regrowth surface
can be prevented.
According to a fifth aspect of the present invention, a semiconductor laser
device comprises a first conductivity GaAs substrate, a first conductivity
type Al.sub.x Ga.sub.1-x As (0<x<1) disposed on the substrate, a
quantum-well structure active layer comprising first conductivity type
Al.sub.z Ga.sub.1-z As (0<z<x) barrier layers and Al.sub.y Ga.sub.1-y As
(0<y<z) well layers, and having a region which is disordered by diffusing
the vacancies in the vicinity of the laser resonator facet, disposed on
the lower cladding layer, a second conductivity type Al.sub.r Ga.sub.1-r
As (z<r<1) first upper cladding layer disposed on the quantum-well
structure active layer, a second conductivity type Al.sub.r Ga.sub.1-r As
(z<r<1) second upper cladding layer disposed on the first upper cladding
layer, and a second conductivity type GaAs contact layer disposed on the
second upper cladding layer. Therefore, there is no necessity of
implanting Si ions to disorder the quantum-well structure active layer,
and it is possible to form a window structure by disordering the
quantum-well structure active layer without generating crystal
transitions. In addition, there is no necessity of implanting Si ions so
as to diffuse those to form a window structure, and there arises no
unlikelihood of disordering due to that Si ions are trapped during their
diffusion by a lot of crystal defects which are formed by the ion
implantation, whereby a semiconductor laser device provided with a desired
window structure can be obtained with high reproducibility.
According to a sixth aspect of the present invention, in the semiconductor
laser device, the quantum-well structure active layer is disordered at the
region other than the stripe shaped active region having a prescribed
width extending in the laser resonator direction among the region except
the vicinity of the laser resonator facet by the diffusion of vacancies,
regions of an upper portion of the second upper cladding layer and the
contact layer other than the region on the active region are made of high
resistance by the implantation of protons, and electrodes are disposed on
the rear surface of the GaAs substrate and on an upper surface of the
contact layer. Therefore, there is no necessity of implanting Si ions to
disorder the quantum-well structure active layer, and it is possible to
form a window structure by disordering the quantum-well structure active
layer without generating crystal transitions. In addition, there is no
necessity of implanting Si ions so as to diffuse those to form a window
structure, whereby a semiconductor laser device provided with a desired
window structure can be obtained with high reproducibility.
According to a seventh aspect of the present invention, in the
semiconductor laser device, a GaAs surface protecting layer is inserted
between the second conductivity type first upper cladding layer and the
second conductivity type second upper cladding layer, thereby preventing
the surface roughness of the regrowth surface.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a)-1(c) are diagrams illustrating a semiconductor laser device
according to a first embodiment of the present invention.
FIGS. 2(a)-2(e) are diagrams illustrating process steps in a method of
fabricating a semiconductor laser device according to the first embodiment
of the present invention.
FIGS. 3(a)-3(b) are diagrams illustrating main process steps in a method of
fabricating a semiconductor laser device according to a second embodiment
of the present invention.
FIG. 4 is a sectional view illustrating a main process step in a method of
fabricating a semiconductor laser device according to a third embodiment
of the present invention.
FIGS. 5(a)-5(c) are diagrams illustrating a prior art semiconductor laser
device.
FIGS. 6(a)-6(e) are diagrams illustrating process steps in a method of
fabricating a prior art semiconductor laser device.
FIGS. 7(a)-7(b) are graphs showing aluminum profiles for explaining
disordering of a quantum-well structure active layer in the prior art
semiconductor laser device.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
FIG. 1(a) is a perspective view illustrating a structure of a semiconductor
laser device according to a first embodiment of the present invention,
FIG. 1(b) is a cross-sectional view taken along a line 1b--1b of FIG.
1(a), i.e., in the laser resonator length direction of FIG. 1(a), and FIG.
1(c) is a cross-sectional view taken along a line 1c--1c of FIG. 1(a),
i.e., in the direction perpendicular to the laser resonator length
direction of FIG. 1(a). In FIGS. 1(a)-1(c), reference numeral 1 designates
an n type GaAs substrate. An n type Al.sub.x Ga.sub.1-x As (x=0.5) lower
cladding layer 2, 1.5.about.2.0 .mu.m thick, is disposed on the n type
GaAs substrate 1. A quantum-well structure active layer comprising three
Al.sub.y Ga.sub.1-y As (y=0.05.about.0.15) well layers (not shown) about
10 nm thick and two Al.sub.z Ga.sub.1-z As (z=0.2.about.0.35) barrier
layers (not shown) about 10 nm thick alternatingly laminated with each
other, and further provided with light guide layers (not shown) about 35
nm thick having the same composition as the barrier layers at the both
sides of the quantum well structure, is disposed on the lower cladding
layer 2. Reference numeral 4a is a p type Al.sub.r Ga.sub.1-r As (r=0.5)
first upper cladding layer and numeral 4b is a p type Al.sub.r Ga.sub.1-r
As (r=0.5) second upper cladding layer, respectively, in which the total
thickness of the first upper cladding layer 4a and the second upper
cladding layer 4b is about 1.5.about.2.0 .mu.m. Reference numeral 5
designates a p type GaAs contact layer 0.5.about.1.0 .mu.m thick and
numeral 8 designates a proton implanted region. Reference numeral 9
designates an n side electrode and numeral 10 designates a p side
electrode. Reference numeral 6 designates a vacancy diffusion region,
numeral 20 designates a laser resonator facet, numeral 3a designates an
active region contributing to the laser light emission of the active layer
3, and numeral 3b designates a window structure region formed in the
vicinity of the laser resonator facet 20 of the active layer 3. In
addition, the semiconductor laser device is 300.about.600 .mu.m long in
the resonator length direction and 300 .mu.m in width.
FIGS. 2(a)-2(e) are diagrams illustrating process steps in a method of
fabricating a semiconductor laser device according to a first embodiment
of the present invention. In the figures, the same reference numerals as
in figures 1(a)-1(c) designate the same or corresponding parts. Reference
numeral 16 designates an SiO.sub.2 film, numeral 16a designates a stripe
shaped opening extending in what becomes a laser resonator length
direction and disposed on the SiO.sub.2 film 16, numeral 17 designates a
photoresist, and arrows 23 indicate proton implantations, respectively.
A description is given of the fabricating method. Initially, the n type
Al.sub.x Ga.sub.1-x As (x=0.5) lower cladding layer 2, the quantum-well
structure active layer 3, and the p type Al.sub.r Ga.sub.1-r As (r=0.5)
first upper cladding layer 4a are successively epitaxially grown on the n
type GaAs substrate 1 which is in a wafer state. For this crystal growth
method, metal organic chemical vapor deposition (hereinafter, referred to
as MOCVD) or molecular beam epitaxy (hereinafter, referred to as MBE) each
having high controllability for the film thickness are employed. FIG. 2(a)
shows a cross-section of the wafer after the growth.
The surface of the p type first upper cladding layer 4a is covered with the
SiO.sub.2 film 16, having a stripe shaped opening 16a extending in the
laser resonator length direction and having a length not reaching a
position of what becomes the laser resonator facet on what becomes an
active region, as shown in FIG. 2(b). The SiO.sub.2 film 16 is formed by
plasma CVD (chemical vapor deposition) or sputtering. The thickness of the
SiO.sub.2 film 16 is preferably 1000 .ANG. and the stripe width of the
opening 16a is preferably 1.about.5 .mu.m. The interval between the
opening and the position of what becomes the laser resonator facet is
preferably about 20 .mu.m. Next, the wafer is annealed at a temperature
above 800.degree. C. The annealing is performed in an ambient with As
pressure applied in order to prevent escape of As from the opening 16a.
As described in Applied Physics Letters, vol. 52, 1988, pp 1511-1531, the
SiO.sub.2 film can absorb Ga atoms from the GaAs or AlGaAs crystal during
annealing, so that Ga atoms escape out from the surface of the p type
first upper cladding layer 4a adjacent a region other than the opening 16a
of the SiO.sub.2 film 16, thereby generating vacancies where Ga atoms are
missing from lattice positions of the p type first upper cladding layer 4a
where Ga atoms are originally present. When the vacancies are further
diffused into the semiconductor crystal by annealing to reach the
quantum-well structure active layer 3, disordering of the quantum-well
structure occurs. Therefore, in the active layer 3 directly below the
SiO.sub.2 film 16, Ga atoms escape to form vacancies and disordering of
the quantum-well structure occurs and the vacancies are diffused, thereby
increasing effective band gap energy in this region. As a result, the
regions in the vicinity of the laser resonator facets of the disordered
regions serve as window structure parts 3b functioning as "window" for the
laser beam that is emitted from the active region 3a of the active layer 3
that is positioned under the SiO.sub.2 film opening 16a and is not
disordered. In addition, the thickness of the p type first upper cladding
layer 4a is preferably less than 0.5 .mu.m because the distance along
which the Ga vacancies efficiently diffuse is less than 0.5 .mu.m.
After the annealing, the SiO.sub.2 film 16 is removed by wet etching, and
the p type second upper cladding layer 4b and the p type contact layer 5
are epitaxially grown on the p type first upper cladding layer 4a.
Thereafter, the surface of the contact layer 5 is covered with the resist
film and it is patterned by photolithography, and then in the step of FIG.
2(e), a stripe shaped resist 17 extending in the laser resonator length
direction and having approximately the same size as the opening 16a is
formed on a region where the opening 16a of the SiO.sub.2 film 16 was
formed, and proton implantation 23 is carried out from above the contact
layer 5, employing the resist 17 as a mask, so that the implantation peak
is positioned in the p type second upper cladding layer 4b and the
concentration of the protons is 4.times.10.sup.19 cm.sup.-3. As a result,
regions 8 where implantation of protons is carried out are formed on the
contact layer 5 and the second upper cladding layer 4b except below the
resist 17. These proton implantation regions 8 are high resistance regions
and function as current blocking layers.
At last, a p side electrode 10 is formed on the contact layer 5 and an n
side electrode 9 is formed on the rear surface of the GaAs substrate 1,
and a pair of laser resonator facets 20 are formed by cleaving the wafer,
resulting in a semiconductor laser device.
A description is given of the operation. When a voltage is applied with the
p side electrode 10 a plus voltage and the n side electrode 9 a minus
voltage, holes are injected, passing through the p type contact layer 5,
the p type second upper cladding layer 4b, and the p type first upper
cladding layer 4a into the quantum-well structure active layer 3, and
electrons are injected, passing through the n type semiconductor substrate
1 and the n type AlGaAs cladding layer 2 into the quantum-well structure
active layer 3. Then, radiative recombination of electrons and holes
occurs in the active region of the active layer 3, thereby generating
induced emission of light in the active region 3a comprising the
quantum-well structure active layer 3. When light having a gain higher
than the loss of the waveguide is generated by increasing sufficiently the
injected quantity of carriers, the laser oscillation occurs. Here, since
the regions 8 where protons are implanted have a high resistance, no
current flows through the p type contact layer 5 and the p type second
upper cladding layer 4b in these regions 8. In other words, a current
flows only through the region where no protons are implanted.
Since in the semiconductor laser device of this first embodiment the active
layer 3 in the vacancy diffusion region 6 is disordered by the diffusion
of vacancies, the band gap energy thereof is larger than that in the
region which is not disordered, of the active region 3a, whereby the
region in the vicinity of the laser resonator facet 20 of the disordered
region functions as a window structure region 3b which does not absorb the
laser light. Further, since the region adjacent the active region 3a,
perpendicular to the laser resonator length direction, of the active layer
3 is also disordered, a refractive index distribution is produced in the
direction perpendicular to the laser resonator length direction in the
active layer 3, whereby the laser beam is confined in the active region 3a
and is guided in the laser resonator length direction.
In the first embodiment, annealing is carried out with the SiO.sub.2 film
16 disposed on the surface of the p type Al.sub.r Ga.sub.1-r As (r=0.5)
first upper cladding layer 4a, whereby vacancies are formed in the p type
first upper cladding layer 4a and the quantum-well structure active layer
3 is disordered with the diffused vacancies. Therefore, implantation of Si
ions is not required for the disordering as in the prior art, and a lot of
crystal defects which would be otherwise generated due to Si ions having
high energy colliding with the crystals at the ion implantation can be
avoided. Thereby, generation of crystal transitions can be suppressed and
the disordered region can be prevented from failing to function as a
window structure due to the laser beam being absorbed by the crystal
transitions as has been a problem in the prior art. Therefore, it is
possible to obtain a high light output operation from a semiconductor
laser device with a window structure, that exhibits superior device
characteristics with a high facet destruction level and high reliability.
Since there is no implanting process of Si ions with high energy as in the
prior art, the amount of the crystal defects generated can be
significantly decreased, and there arises no unlikelihood of disordering
due to Si ions being trapped by a lot of crystal defects by diffusing the
vacancies instead of Si for disordering the active layer 3, a
semiconductor laser device with a desired window structure can be obtained
with high reproducibility.
According to the first embodiment, annealing is carried out with the
SiO.sub.2 film 16 disposed on the surface of the p type Al.sub.r
Ga.sub.1-r As (r=0.5) first upper cladding layer 4a, whereby vacancies are
formed in the p type first upper cladding layer 4a and the quantum-well
structure active layer 3 is disordered by the diffused vacancies.
Therefore, a window structure with small number of crystal transitions can
be formed by disordering the active layer 3 without implanting Si ions and
a semiconductor laser device provided with such a desired window structure
can be obtained with high reproducibility.
Embodiment 2
FIG. 3(a) is a perspective view illustrating a main process step in a
method of fabricating a semiconductor laser device according to a second
embodiment of the present invention and FIG. 3(b) is a sectional view
taken along a line 3b--3b of FIG. 3(a). In FIGS. 3(a)-3(b), the same
reference numerals as in FIGS. 2(a)-2(e) designate the same or
corresponding parts. Reference numeral 19 designates an Si.sub.3 N.sub.4
film. In the process of diffusing the vacancies in accordance with the
first embodiment, after an SiO.sub.2 film 16 having an opening 16a is
formed, a region including the opening 16a is covered with the Si.sub.3
N.sub.4 film 19 to perform the annealing as shown in FIG. 3(a). Other
process steps are the same as in the first embodiment.
In the first embodiment, the p type Al.sub.r Ga.sub.1-r As (r=0.5) first
upper cladding layer 4a is exposed at the stripe shaped opening 16a formed
in the SiO.sub.2 film 16. Even though the annealing is performed under
environment where the As pressure is applied, if the crystal surface is
exposed during the annealing when the vacancies are diffused, escape of As
atoms from the surface is not perfectly suppressed and there is generated
surface roughness at the p type first upper cladding layer 4a in the
opening 16a. In addition, there is a tendency that this surface roughness
becomes worse as the Al composition ratio is higher. When surface
roughness is generated, crystal transitions arise during crystal regrowth
on the p type first upper cladding layer 4a having a rough surface, and
the crystal transitions enter into the active layer 3, thereby
deteriorating the operating characteristics and reliability, resulting in
difficulty in producing a high quality semiconductor laser device. In this
second embodiment, the p type Al.sub.r Ga.sub.1-r As (r=0.5) first upper
cladding layer 4a in the opening 16a is not exposed by covering the region
including the opening 16a with the Si.sub.3 N.sub.4 film 19 as shown in
FIGS. 3(a)-3(b), whereby surface roughening of the p type Al.sub.r
Ga.sub.1-r As (r=0.5) first upper cladding layer 4a during annealing can
be prevented. In addition, after the annealing, the Si.sub.3 N.sub.4 film
19 is removed together with the SiO.sub.2 film 16 by dry etching employing
CF.sub.4 and wet etching comprising hydrofluoric acid.
According to the second embodiment, since the annealing is performed after
the region including the opening 16a of the SiO.sub.2 film 16 disposed on
the p type Al.sub.r Ga.sub.1-r As (r=0.5) first upper cladding layer 4a is
covered with the Si.sub.3 N.sub.4 film 19, the surface roughening of the p
type Al.sub.r Ga.sub.1-r As (r=0.5) first upper cladding layer 4a exposed
on the opening 16a of the SiO.sub.2 film 16 due to escape of As atoms is
suppressed, thereby producing a semiconductor laser device with high
operating characteristic and high reliability.
Embodiment 3
FIG. 4 is a sectional view perpendicular to the laser resonator length
direction, illustrating a main process step in a method of fabricating a
semiconductor laser device according to a third embodiment of the present
invention. In FIG. 4, the same reference numerals as in FIGS. 2(a)-2(e)
designate the same or corresponding parts. Reference numeral 13 designates
a GaAs surface protecting layer. In the third embodiment, after the p type
Al.sub.r Ga.sub.1-r As (r=0.5) first upper cladding layer 4a is formed as
shown in FIG. 2(a), the p type GaAs protecting layer 13 is further
subsequently epitaxially grown, the SiO.sub.2 film 16 including the
opening 16a at the surface of the p type GaAs protecting layer 13 is
formed, and the annealing is performed to make Ga escape to form vacancies
as well as to diffuse those vacancies. Other process steps are the same as
in the first embodiment.
In the first embodiment, the SiO.sub.2 film 16 is epitaxially grown on the
p type Al.sub.r Ga.sub.1-r As (r=0.5) first upper cladding layer 4a and
the active layer 3 is disordered by the annealing to form vacancies as
well as to diffuse the vacancies and, thereafter, the SiO.sub.2 film 16 is
removed and the second upper cladding layer 4b and the contact layer 5 are
epitaxially grown. However, the first upper cladding layer 4a having the
crystal regrowth surface includes a large quantity of Al atoms, and this
layer is easily oxidized. Accordingly, when the first upper cladding layer
4a is exposed to the air in the step of forming the SiO.sub.2 film 16,
surface roughness is generated by oxidation, and there occur crystal
transitions at the regrowth interface. In addition, the crystal
transitions are broadened at the active layer 3 and the performance of the
semiconductor laser device is deteriorated because the laser beam is
absorbed by the crystal transitions.
In the third embodiment, the GaAs surface protecting layer 13 is further
successively formed on the first upper cladding layer 4a which is
epitaxially grown, the SiO.sub.2 film 16 is formed on the GaAs surface
protecting layer 13, and the annealing is performed to form vacancies as
well as to diffuse those vacancies, thereby disordering the active layer
3. Thereafter, when the SiO.sub.2 film 16 is removed and the second upper
cladding layer 4 and the contact layer 5 are again epitaxially grown on
the GaAs surface protecting layer 13, the GaAs surface protecting layer 13
having a regrowth interface does not include Al atoms and this layer is
hard to oxidized, whereby surface roughness due to oxidation is suppressed
and the crystal transitions at the regrowth interface are reduced. In
addition, since the GaAs surface protecting layer 13 may absorb the laser
light dependent on the composition and the structure of the active layer
3, the thickness of the GaAs surface protecting layer 13 is preferred to
be a thickness that has no effects on the laser characteristic, i.e., a
thickness less than 100 .ANG..
According to the third embodiment, after the GaAs surface protecting layer
13 is formed on the p type Al.sub.r Ga.sub.1-r As (r=0.5) first upper
cladding layer 4a following the epitaxial growth of the first upper
cladding layer 4a, an SiO.sub.2 film 16 is formed and annealing is
performed to form vacancies and to diffuse the vacancies. Therefore,
generation of surface roughness of the regrown interface due to oxidation
can be suppressed, and the crystal transitions at the regrown interface
can be reduced, whereby a semiconductor laser device with high quality
operating characteristics and high reliability is obtained.
In the above-described embodiments, the semiconductor laser device having a
structure in which the contact layer 5 and the upper parts of the first
upper cladding layer 4a and the second upper cladding layer 4b on the
region except the active region 3a of the active layer 3 are made of high
resistivity by proton implantation is described. In the present invention,
a semiconductor laser device having another structure such as a ridge
structure may be used. Even in this case, the vacancies are diffused in
the vicinity of the laser resonator facet, whereby the quantum-well
structure active layer 3 is disordered, with the same effects as in the
above-described embodiments.
While in the above-described embodiments the n type GaAs substrate is
employed as the semiconductor substrate 1, a p type GaAs substrate may be
employed in the present invention, with the same effects as in the
above-described embodiments.
While in the above-described embodiments the active layer 3 has a
multi-quantum-well structure (MQW), another quantum-well structure such as
a single-quantum-well structure (SQW) may be used as the active layer 3 in
the present invention, with the same effects as in the above-described
embodiments.
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